Heat-sensitive sheet for thermographic copying



July 12, 1966 E. c. OTTO 3,260,613

HEAT-SENSITIVE SHEET FOR THERMOGRAPHIC COPYING Filed March 15, 1963DISPERSED PHENYL THIOUREA DISPERSED LEAD COMPOUND CHLORINATED PARAFFINWAX BINDER United States Patent 3,260,613 HEAT-{SENSITIVE SHEET FORTHERMO- GRAPHIC COPYING Edward C. Otto, Brooklyn, N.Y., assignor toInterchemical Corporation, New York, N.Y., a corporatron of Ohio FiledMar. 15, 1963, Ser. No. 265,337 14 Claims. (Cl. 117-36.9)

This invention relates to copy sheets and to a method of making copiestherewith. More particularly the invention relates to copy sheets foruse in making copies by a thermographic technique wherein the image onthe copy sheet is the result of a chemical change produced by localizedheating in the image area.

In recent years thermocopying of typed or printed originals has becomeincreasingly popular. Thermographic copying involves placing theoriginal in heat conductive relationship with a sheet of copy paper andthen subjecting the original and copy sheet to intense irradiation ofsuch character that the printed or type matter on the original absorbsthat least a portion of the radiant energy thereby converting it intoheat which causes the image of the original to be developed in the copysheet. In such a process, it is apparent that the copy sheet must beconstructed in such manner as to be capable of changing to a contrastingcolor when heated. The thermographic copying process machines forcarrying out the process and certain types .of heat-sensitive papers aredescribed in numerour prior art patents, for instance, United StatesPatents 2,663,654-7; 2,710,263; 2,740,895-6.

The .heat induced image may be formed either by a physical change in thecopy sheet or by a chemical reaction induced by the heat. This inventionis concerned with heat-sensitive sheets in which the image is formed asa result of a chemical reaction.

A typical thermogra-phic copying paper in which image formation resultsfrom a chemical reaction is the copying paper described in U.S. Patent2,663,657. This paper has a coating of two materials that are reactivewith each other to produce a dark colored product which forms the image.The reactants are disperse as discrete particles in a binder. At leastone of the reactants is fusible within the range of 60 to 120 C. Thus,when the coating is heated to a temperature within said range, thefusible reactant particles melt and flow into contact with the particlesof the other reactant with the resultant reaction. US. Patent 2,663,657further states that the binder should be infusible within the 60 to 120range in order to avoid undesirable sticking of the copying paper tocontacting surfaces during copying.

I have now discovered a novel thermographic copying paper of thechemically reactive type in which both of the reactants are infusiblewithin the 60 to 120 C. range and which employs a binder which isfusible within the 60 to 120 C. range. Furthermore the use of the binderof this invention creates none of the problems of undesirable stickinesswhich eliminated the use of low melting binders in US. Patent No.2,663,657

The paper of this invention may be generally described as aheat-sensitive copying sheet comprising a base sheet, preferably ofinfrared transparent paper, carrying a coating comprising individualdispersions of two reactants which are capable of reacting to formcolored sulfides of heavy metal. One of the reactants may be describedas the heavy metal donor and the other reactant as the sulfide donor.The reactants are of very low water solubility and preferably aresubstantially insoluble in water. Both reactants have fusion pointsgreater than 120 C. The reactants are dispersed in a binder which has afusion point of from 60 to 120 C. Accordingly, the binder "Ice meltsduring the copying operation and carries the indivrdual dispersedparticles of the reactants into contact with each other where they mayreact.

While other heavy metal salts, notably mercury, silver, cadmium, nickel,cobalt and tin have been used with adequate results in this invention,the heavy metal donor is preferably a lead salt, having a fusion pointgreater than C., lead hydroxide or a lead oxide compound. The lead saltsmay be the salt of any acid except strong acids such as sulfuric,hydrochloric or nitric acids. Suitable lead salts include lead maleate,lead carbonate, lead formate and lead phosphate. The term lead oxide ismeant to include lead oxide containing compounds e.g., the commerciallyavailable compounds:

Lectro 7 8-tetrabasic lead fumarate,

4Pb0 -PbC- H (COO 2H O Tribase-tribasic lead sulfate,

3Pb0- PbSO -H O Dy-thaldibasic or lead phthalate,

The sulfur donor is preferably a thiourea compound and most preferably asubstituted thiourea having a fusion point above 120 C. and beingsubstantially insoluble in water. Two such substituted thioureas whichare very suitable for the practice of this invention are monophenylthiourea and diphenyl thiourea.

The binder is preferably a chlorinated paraflin wax containing at least50% chloride and having a fusion point between 60 andl20. (It should benoted that in the present specification and claims, all proportions areby weight unless otherwise stated.) I have found such colorinatedparaffin wax binders display stability or aging characteristics farsuperior to other binders having the requisite 60 to 120 C. fusionpoint. Representative of the binders having the requisite melting pointswhich were tried were ethyl cellulose polymer, chlorinated naturalrubber having a 67% chlorine content and polyamides formed by thecondensation polymerization of dilinolenic acid and ethylene diamine.These binders produced papers which had a lower stability and a lowerresistance to aging as compared to the papers of this invention. I havefurther found that, in some instances, where the images on copy arelighter than desired, the darkness of the image may be enhanced by theaddition to the binder of small amounts of a wax which has a lowerviscosity than the chlorinated paraffin wax. Microcrystalline wax is anexcellent additive for this purpose as is parafiin wax. When used, thewax additive preferably constitutes from 15% to 30% of the total binderweight.

Preferably a sufficient amount of the lead donor is used to provide from1 to 4 sulfur atoms for each lead atom. Preferably, from 0.2 to 2.0parts of binder are used for each part of total reactants.

In preparing the paper, the reactants are applied to the substrate asdispersions in a volatile organic solvent solution of the binder. Thecoating may be applied in a single application. However, as thereactants display some tendency to react with each other while dispersedin these solutions, the preferable technique is to apply the coatinglayer in two steps. One of the reactants dispersed in the bindersolution is first applied after which the second reactant in the samebinder solution is applied. The

use of this technique also permits the manufacture of a formulaC24H29Cl21) 7 Lead carbonate 8 Microcrystalline wax (M.P. about 90 C.,Fisher- Johns method) 3 Toluene 15 It has been further found that insome instances even further freedom from tack may be obtained by theinclusion of a filler. Best results have been obtained using mica orcalcium carbonate as fillers. However any other conventional colorlessor white filler having a melting point above 120 C. may be used. Whenpresent, the filler preferably constitutes from 10 to 40% of the totalbinder present. Where the coating is applied in two applications, thefiller is included preferably in the second layer applied so that itWill 'be predominantly in the upper part of the coating.

Then, 15 additional parts of toluene are added and the mixture isapplied to an infrared transparent White paper at a coating thickness of0.1 mil using a No. 3 Mayer rod. The coating is then permitted to dry atambient temperature for 20 seconds.

The applied coating is then overcoated with a composition prepared bythe same procedure as above containmg:

' Parts by Wt. Chlorowax 70 6 Monophenyl thiourea Microcrystalline wax(M.P. 90 C.) 1.5

Toluene The resulting coating on the paper is uniform, has good adhesionto the paper and excellent strain and pressure resistance. In additionthe paper has excellent resistance to aging. When subjected to theaforementioned accelerated aging test for 72 hours at 120 -F. and 97%relative humidity the paper still produces readable copies.

When exposed in the Thermo-Fax copying machine by either front or backprinting techniques with an original, the sheet produces a clear copy inwhich the graphic matter is black against a white background.

Example 2.The following batches A and B are prepared separately bymixing on a ball mill in accordance with the procedure of Example 1.Batch A:

Chlorowax 70 Tribasic lead maleate monohydrate having the for- Parts bywt. 6.0

Monophenyl thiourea 5.0 Microcrystalline wax 1.5 Toluene 30.0

The two batches are then mixed and immediately coated on an infraredtransparent base paper using a No. 3 Mayer coater. The resulting coatingis uniform, has good adhesion to the paper and excellent strain andpressure resistance. When exposed, after drying, to the Thermo-Faxcopying machine by either front or back printing methods with anoriginal, the sheet produces a clear copy in which the graphic matter isdark brown against a white background.

Example 3.Using the procedure of Example 1 which involves two steps inthe application of the paper coating, a coated paper is produced fromthe following compositions:

Step 1 composition: Parts by wt. Chlorowax 70 6.0 Tribasic lead maleatemonohydrate 7.0 Microcrystalline wax 1.5 Toluene 30.0

Step 2 composition:

Chlorowax 70 6.0 Monophenyl thiourea '5-.0 Microcrystalline wax 1.5Toluene 30:0

The resulting coated paper has all of the properties of the paper ofExample 1 and in addition has greater heatsensitivity.

Example 4.Using the 2 step procedure of Example 1. a coating is producedfrom the following compositions:

Step 1 composition: Parts by wt; Chlorowax 70 6.0 Tribasic lead maleate7.0 Microcrystalline wax 1.5 Toluene 30.0

Step 2 composition: Chlorowax 70 7.0 Diphenyl thiourea 5.0

Toluene 30.0

Chlorowax 70 6.0 Monophenyl thiourea 5.0 1.5

Microcrystalline wax v Toluene 30.0

The resulting coated paper has all of the desirable properties of thepaper of Example 1. 7

Example 6.Using the 2 step procedure of Example 1; a coating is producedfrom the following compositions:

Step 1 composition: Parts by wt. Chlorowax 70 7.0 Lead hydroxide 5.0

Microcrystalline Wax Toluene 30.0 Step 2 composition: Chlorowax 70 7.0Diphenyl thiourea 5.0 Toluene 30.0

The resulting coated paper has all of the desirable properties of thepaper of Example 1.

Example 7 .-Example 5 is repeated using the same conditions, ingredientsand proportions except that lead maleate is used in place of leadhydroxide. The results are the same as in Example 5.

Example 8.--Example 5 is repeated using the same conditions, ingredientsand proportions except that lead oxide is used in place of leadhydroxide. The results are the same as in Example 5.

Example 9.-Example 3 is repeated using the same ingredients, proportionsand conditions except that in place of the Chlorowax 70 binder in thecompositions of both step 1 and step 2, there is used a combination of4.0 parts of Chlorowax 70 and 2 parts of Chlorowax 50 in each step.(Chlorowax 50 is chlorinated paraflin having a chlorine content of 50%and a molecular Weight of about 660.) The resulting paper had all of thedesirable properties of the paper of Example 3 except that the paper ofExample 3 had slightly better stability on accelerated aging.

Example 10.-Example 1 is repeated using the same ingredients, conditionsand proportions except that, the second applied coating further includes5 parts of mica. The resulting paper has all of the desirable propertiesof the paper of Example 1.

While there have been described what is at present considered to be thepreferred embodiments of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

What is claimed is 1. A heat-sensitive copying sheet adapted, on beingplaced in heat-conductive relationship with a. graphic original havinggraphic representations highly absorbtive of infrared rays and onirradiation of said graphic original with infrared rays, to produce acopy of said graphic original, said copying sheet comprising a basesheet having thereon a coating comprising (A) a dispersion of a leadcompound having a melting point above 120 C. selected from the groupconsisting of lead salts of maleic acid, lead carbonate, lead phosphite,lead formate, lead hydroxide and lead oxide and (B) a dispersion ofphenyl substituted thiourea having a melting point above 120 C. in

(C) a binder comprising a major portion of chlorinated paraflin waxhaving a chloride content of at least 50% by weight and a melting pointbetween 60 and 120 C.

2. The copying sheet of claim 1 wherein said lead compound is a leadsalt of maleic acid.

3. The copy sheet of claim 2 wherein said lead salt is tribasic leadmaleate monohydrate.

4. The copying sheet of claim 1 wherein said lead compound is leadcarbonate.

5. The copying sheet of claim 1 wherein said lead compound is leadoxide.

6. The copying sheet of claim 1 wherein said lead compound is leadphosphite.

7. The copying sheet of claim 1 wherein said thiourea is monophenylthiourea.

8. The copying sheet of claim 1 wherein said thiourea is diphenylthiourea.

9. The copying sheet of claim 1 wherein the binder further comprisesmicrocrystalline Wax blended with the chlorinated paraffin wax.

10. The heat-sensitive copying sheet of claim 1 wherein the dispersionof the lead compound is situated predominantly in the lower half of thecoating layer and the dispersion of the phenyl substituted thiourea issituated predominantly in the top half of the coating layer.

11. A copying sheet according to claim 1 comprising a dispersion oftribasic lead maleate monohydrate and of monophenyl thiourea in a bindercomprising a chlorinated paraflin wax having a chlorine content of aboutby weight.

12. A copying sheet according to claim 1 comprising a dispersion oftribasic lead maleate monohydrate and of monophenyl thiourea in a bindercomprising a paraffin wax having a chlorine content of about 70% byweight and a microcrystalline wax.

13. The copying sheet of claim 1, further including a colorless fillerhaving a melting point above C.

14. The copying sheet of claim 13 wherein said filler is mica.

References Cited by the Examiner UNITED STATES PATENTS 2,625,494 1/ 1953Morrison 11736.8 2,916,395 12/1959 Owen 11736.8 2,967,784 1/ 1961 Newmanet al 11736.8 2,999,035 9/1961 Sahler 117-36.8 3,080,254 3/1963 Grant1l7-36.8 3,107,174 10/ 1963 Wartman 117-368 3,111,423 11/1963 OStie117-369 MURRAY KATZ, Primary Examiner.

WILLIAM D. MARTIN, Examiner.

1. A HEAT-SENSITIVE COPYING SHEET ADAPTED, ON BEING PLACED INHEAT-CONDUCTIVE RELATIONSHIP WITH A GRAPHIC ORIGINAL HAVING GRAPHICREPRESENTATIONS HIGHLY ABSORBTIVE TO INFRARED RAYS AND ON IRRADIATION OFSAID GRAPHIC ORIGINAL WITH INFRARED RAYS, TO PRODUCE A COPY OF SAIDGRAPHIC ORIGINAL, SAID COPYING SHEET COMPRISING A BASE SHEET HAVINGTHEREON A COATING COMPRISING (A) A DISPERSION OF A LEAD COMPOND HAVING AMELTING POINT ABOVE 120*C. SELECTED FROM THE GROUP CONSISTING OF LEADSALTS SALTS OF MALEIC ACID, LEAD CARBONATE, LEAD PHOSPHITE, LEADFORMATE, LEAD HYDROXIDE AND LEAD OXIDE AND (B) A DISPERSION OF PHENYLSUBSTITUTED THIOUREA HAVING A MELTING POINT ABOVE 120*C. IN (C) A BINDERCOMPRISING A MAJOR PORTION OF CHLORINATED PARAFFIN WAX HAVING A CHLORIDECONTENT OF AT LEAST 50% BY WEIGHT AND A MELTING POINT BETWEEN 60* AND120*C.